

struct ListNode
{
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};

class Solution
{
public:
    ListNode *swapPairs(ListNode *head)
    {
        if (!head || !head->next)
            return head;
        ListNode *left = head, *right = head->next;
        ListNode *pre = nullptr;
        while (left && right)
        {
            ListNode *nextL = right->next;
            ListNode *nextR = nextL ? nextL->next : nullptr;
            if (!pre)
                head = right;
            else
                pre->next = right;
            right->next = left;
            left->next = nextL;
            pre = left;
            left = nextL, right = nextR;
        }
        return head;
    }
};